1. Field of the Invention
This invention relates generally to image pickup apparatuses and more particularly to an image pickup apparatus using a solid-state image pickup device (or imager), in which a light-receptive region is divided into a plurality of areas so that an image signal is output from each of the light-receptive areas.
2. Description of the Related Art
FIG. 6A illustrates an example of a structure of a CCD (charge-coupled device) imager. The example of FIG. 6A has a light-receptive region (area) 10 undivided. A signal charge corresponding to each picture element (or pixel) that is transferred vertically (in parallel) from the light-receptive area 10 to horizontal charge-transfer CCDs 12 is then transferred horizontally (serially) through the horizontal charge-transfer CCDs 12 to an output amplifier 14 in which the signal charge is amplified and output from a single image output terminal 16.
The number of horizontal charge-transfer CCDs 12 provided for the imager is several to several tens more than the number of effective picture elements along the horizontal length of the light-receptive area 10, to provide a period for stabilizing the horizontal charge-transfer CCDs 12 and the output amplifier 14 before starting the output of image signals corresponding to effective picture elements. This is because early-stage performance characteristics shown at startup of the horizontal charge-transfer CCDs 12 and the output amplifier 14 that have been at rest during a horizontal blanking period are worse than stable-stage performance characteristics thereof shown during stable operation. In other words, the above arrangement is designed for the purpose of preventing the inferior early-stage performance characteristics from affecting the output of image signals corresponding to the effective picture elements.
FIG. 6B illustrates another example of the structure of the CCD imager. The example of FIG. 6B has a light-receptive region divided into three areas to improve a data rate of output signals of the imager. Signal charges corresponding to picture elements that are transferred vertically from light-receptive areas 10a, 10b and 10c to horizontal charge-transfer CCDs 12a, 12b and 12c, respectively, are then transferred horizontally through the horizontal charge-transfer CCDs 12a, 12b and 12c to output amplifiers 14a, 14b and 14c, respectively, in which the signal charges are amplified and output from three image output terminals 16a, 16b and 16c, respectively.
In the imager as shown in FIG. 6B, the light-receptive areas 10a, 10b and 10c are arranged with no gap left therebetween, and thus it is difficult to provide an extra number of the horizontal charge-transfer CCDs 12a, 12b or 12c so as to make the total number thereof exceed the number of the effective picture elements corresponding thereto. As a result, at startup of the horizontal charge-transfer CCDs 12a, 12b and 12c, the output of image signals from the image output terminals 16a, 16b and 16c starts without preliminary stages. Therefore, the early-stage performance characteristics of the horizontal charge-transfer CCDs 12a, 12b and 12c and the output amplifiers 14a, 14b and 14c would adversely affect the image signals, so that ringing and defects would appear in the image signals output immediately after the beginning of a horizontal effective image period for which the image signals corresponding to the effective picture elements are output.
Further, the output signal of the imager has limitations in band imposed by a low-pass filter and an amplifier circuit in a later step performed in an image signal processing circuit, and thus an adequate signal level of the first picture element could not be obtained immediately after the beginning of the effective image period. Accordingly, when the image signals output from respective areas of the divided light-receptive region are combined to constitute one complete image, vertically-striped noise, which could be generated in picture elements located at the junctures between adjoining light-receptive areas, would disadvantageously make it difficult to produce a seamless image.
To relieve the above problems, it has been proposed that an improved design should be applied to the structure of the imager having a divided light-receptive region (see Morinaka et al., “Development of eight-channel parallel reading CCD”, Technical Report of the Institute of Image Information and Television Engineers, Vol. 25, No. 3, pp. 7-11, 2002).
Another approach that has been proposed is that ringing or defects that would appear in the image signals output immediately after the beginning of the horizontal effective image period should be corrected in post-processing (see Mitani et al., “Study of Super-High-Definition Color Image Pickup Experimental Equipment”, Winter Convention , 2000 , the Institute of Image Information and Television Engineers, 4-1, p.71).
However, even if the above approaches were adopted, the image signals corresponding to picture elements located at the junctures between adjoining light-receptive areas would be susceptible to detrimental effects of sample-and-hold pulses and other operations in subsequent processes performed in various circuits. These effects would be highly characteristic of nonlinearity, and thus could not be corrected with ease.
The present invention has been made in view of the above-discussed circumstances, and it is one exemplified object of the present invention to provide an image pickup apparatus that can reduce noise that would be produced in picture elements located at the junctures between adjoining light-receptive areas, to obtain image signals adequate to produce a seamless image, and that can reduce loss of image quality that would be caused by the effects of sample-and-hold pulses and other operations in subsequent processes performed in various circuits.